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OPS4J Base - Utilities/Extensions related to sytem/configuration properties.

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/*
 * Copyright 2008 Alin Dreghiciu.
 *
 * Licensed  under the  Apache License,  Version 2.0  (the "License");
 * you may not use  this file  except in  compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed  under the  License is distributed on an "AS IS" BASIS,
 * WITHOUT  WARRANTIES OR CONDITIONS  OF ANY KIND, either  express  or
 * implied.
 *
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.ops4j.util.property;

/**
 * Resolves properties by first looking at itself and then to a falback resolver.
 *
 * @author Alin Dreghiciu
 * @since 0.5.0, January 16, 2008
 */
public abstract class FallbackPropertyResolver
    implements PropertyResolver
{

    /**
     * Fallback resolver
     */
    private PropertyResolver m_fallbackResolver;

    /**
     * Creates a property resolver with a fallback resolver.
     *
     * @param fallbackResolver rersolver to use to resolve properties
     */
    public FallbackPropertyResolver( final PropertyResolver fallbackResolver )
    {
        m_fallbackResolver = fallbackResolver;
    }

    /**
     * Resolves a property based on its name by first calling the findProperty() and then fallback to falback resolver
     * if property value is null.
     *
     * @param propertyName property name to be resolved
     *
     * @return value of property or null if property is not set or is empty.
     */
    public String get( final String propertyName )
    {
        String value = findProperty( propertyName );
        if( value == null && m_fallbackResolver != null )
        {
            value = m_fallbackResolver.get( propertyName );
        }
        return value;
    }

    /**
     * Resolves a property by name.
     *
     * @param propertyName name of the property to be resolved.
     *
     * @return value of property
     */
    protected abstract String findProperty( String propertyName );

}